1. Field of the Invention
The present invention relates to a scanning optical system and a scanning optical apparatus for performing data recording on a surface to be scanned using the scanning optical system.
2. Related Background Art
Conventionally, a deflector used in a scanning optical system adopts a polygonal mirror in which a deflection velocity of a light beam is equiangular or a galvano mirror (sinusoidal oscillation mirror) in which a deflection velocity of a light beam is not equiangular.
FIGS. 1 to 3 are schematic views showing general arrangements of a scanning optical apparatus using a scanning system. For the sake of simplicity, a description will be made while an optical system for inclination correction is not necessary.
FIG. 1 shows an example wherein a galvano mirror is used as the deflector. A light beam emitted from a light source 101 is collimated to a parallel beam by a collimator lens 102, and the parallel beam is incident on a mirror surface of a galvano mirror 103. The beam is reflected by the mirror surface, passes through an arcsin.theta. lens 104, and forms an image on a surface 106 to be scanned through a cylindrical lens 105. A photosensitive drum 107 for recording the light source is located at the surface 106. The light beam becomes incident on the galvano mirror 103 within a plane perpendicular to the surface 106 and including a pivot axis 108 of the deflector (this plane will be referred to as a z-x plane under an assumption that a coordinate system shown in FIG. 1 is given for the sake of simplicity). The cylindrical lens 105 is inserted for the purpose of correcting a curve of an image surface to be scanned in a z direction during rotation of the mirror surface. Note that in FIG. 1, 109 indicates a driver for the galvano mirror 103.
FIG. 2 shows an example wherein a polygonal mirror is used as the deflector. A light beam emitted from a light source 101 is collimated to a parallel beam by a collimator lens 102, and is incident on a mirror surface of a polygonal mirror 110. The parallel beam is reflected by the mirror surface, and forms an image on a surface 106 to be scanned through an f-.theta. lens 111. The light beam becomes incident on the polygonal mirror 110 within a plane perpendicular to the surface 106 and a pivot axis 108 of the deflector. This plane is called a y-x plane for the sake of descriptive convenience.
FIG. 3 shows another example wherein a polygonal mirror is used as the deflector. A light beam emitted from a light source 101 is collimated into a parallel beam by a collimator lens 102, and is incident on the mirror surface of a polygonal mirror 110. The parallel beam is reflected by the mirror surface, and is imaged onto the surface 106 to be scanned through an f-.theta. lens 111 and a cylindrical lens 105. A laser beam becomes incident on the polygonal mirror 110 within the z-x plane. The cylindrical lens 105 is inserted for the purpose of correcting a curve of an image surface to be scanned in a z direction during rotation of the mirror surface. Note that in FIG. 3, 112 indicates a driver for the polygonal mirror 110.
In the scanning optical apparatus with the above arrangement, a curve of an image surface caused by scanning a light beam upon pivotal movement of the mirror surface of the deflector must be corrected, and a scanning speed of an image on the surface to be scanned by pivotal movement of the deflector must be rendered constant. For this purpose, a plurality of lens systems are required. These lens systems are generally difficult to design, have complicated structures, and hence are expensive. For this reason, the direction of a light beam reflected by the mirror surface is preferably symmetrical within a single plane.
In the example of FIG. 2 of the above-mentioned examples, since the light beam becomes incident on the mirror surface within a plane perpendicular to the surface 106 to be scanned and the pivot axis 108 of the deflector, the reflected light beam is deflected within the same plane. However, the direction of the light beam reflected upon rotation of the polygonal mirror 110 becomes asymmetrical, and the curve of the image surface to be scanned also becomes asymmetrical. Therefore, the structure of a lens system for correcting this is complicated.
When the light beam is incident on the mirror surface from a point within the z-x plane like in FIGS. 1 and 3, symmetricity of the light beam to be reflected can be assured. However, since the image surface of the light beam to be scanned is curved in the z direction upon rotation of the mirror surface, the cylindrical lens 105 must be arranged so as to correct the curve. The cylindrical lens is also used for correcting an inclination error of the mirror surface. The curve in the z direction is larger than the influence caused by an inclination of an angle of the mirror surface, design, manufacture, and adjustment of the cylindrical lens are generally difficult, and this also makes the f-.theta. lens and arcsin.theta. lens difficult to design as well as the cylindrical lens.